The present invention relates to a method for calibrating a position-measuring system and a position-measuring system for detecting a position.
Such position-measuring systems are, for example, used in the field of metrology in order, e.g., to measure structures on a wafer or structures of a mask for producing semiconductor elements. For this, e.g., an image of a structure on a sample is recorded, then the sample (e.g., wafer or mask) moves and the image of a second structure on the sample is recorded. Then, for example, the distance between the two structures as well as a movement distance of the sample can be determined from the images.
Accuracy of such measuring systems must be improved as the structures on the sample becoming steadily smaller. In order to meet these requirements, a movement of the sample may be measured by use of interferometry. Accuracy of the position measurement by the use of interferometry depends on the refractive index of the gaseous medium (e.g., ambient air) in which the sample is arranged. As conventional sensors for determining air pressure and temperature are used, the refractive index of the gaseous medium is continually determined, typically with a known reference section (etalon). To this end, the refractive index is calculated from the changes, measured by use of interferometry, in etalon length signal and known etalon length, and the measured positions are corrected accordingly.
Furthermore, it is known to regulate environmental parameters such as, e.g., temperature, pressure and atmospheric humidity in the measuring area in order to achieve the most constant conditions possible.
However, all these measures are not sufficient to achieve the measurement precision desired when determining the position.